Is it safe to add these to sources.list? I don't want to mess up my system and I know in debian-world that mixing branches could be disastrous. I switched from LMDE to Debian because I had trouble compiling wine from git and figured I'd run into other problems down the line but I really don't like the way XFCE looks in Debian 6.0.4 and if I can safely add debian debs and debian repos then I think I can safely be happy with LMDE. Thoughts?

lmde IS debian squeezy/testing (if you are using latest or incoming, the only difference is a delay in the updates because they are delivered to you in packs in order to be tested and proved saved and breakage-prone)

in one point you are right, tho, mixing releases (stable/testing/sid) can bring instability to your system, but can be done, if you know exactly how and what you are looking for

LMDE is not debian testing. My desktop runs debian testing, the one I'm writing this on runs LMDE. Mine has vlc 2 and latest firefox, this one has not seen an upgrade for months and still pretends to be up to date while running vlc 1.11 and firefox 9.

Debian testing is a rolling release distro, LMDE is more like a fixed cycle update release distro with an additional layer of a mix of useful software (mint tools) and really broken and annoying software (stylish addon, custom google, ...).

Perhaps your LMDE is not Debian testing, but my LMDE has been successfully pointing directly at Debian testing for over a year now. I had not seen the instability and breakages as a serious enough issue, more of a non-issue in my opinion, to start using the update packs last July, so I never have. Still happily running along on testing.

I'm not saying your way of doing LMDE is wrong, just pointing out that LMDE can be Debian testing if you want it to be. Linux, choice, and all that.

You can do it, but you just have to be careful. I use sid to download a few packages every now and then, but I never upgrade with it. (That's a pretty good general rule if you just want to dip your toes in.)